The hormonal form of vitamin D, calcitriol, acts through the vitamin D receptor (VDR) to regulate calcium homeostasis, maintenance of skeletal integrity, immune defense, hormone secretion, hair follicle cycling, and mammary gland development. The overall long-term objective of our studies is to elucidate the pathophysiology of diseases caused by excess or insufficient calcitriol actions. We apply modern imaging, biochemical, and genetic approaches to study the regulation of calcitriol metabolism and actions at the molecular level. Abnormal hormone effects may result from insufficient or excess vitamin D supplementation or from abnormal VDR functions. One of the diseases caused by aberrant VDR functions is hereditary resistance to calcitriol (HVDDR). This disease is usually results from a mutation in the VDR gene and manifests as rickets. We have used skin fibroblasts from subjects with HVDDR to explore the molecular mechanisms of VDR activation. These fibroblasts either lack VDR or express mutant VDRs, which display abnormalities in discrete steps of the receptor activation pathway. We found that several mutations cause defects in VDR subcellular localization and dimerization with retinoid X receptor (RXR). Our first objective was to characterize these defects. We cloned functional fluorescent protein chimeras of VDR and RXR and explored the impact of VDR mutations on VDR functions by site-directed mutagenesis, dynamic microscopy experiments, and nuclear export assays. Microscopy showed that calcitriol and synthetic calcitriol analogues induce rapid receptor redistribution from the cytoplasm into the nucleus. Defects in DNA binding, hormone binding, and dimerization selectively influenced intranuclear compartmentalization and nucleocytoplasmic trafficking of VDR. We identified sequences of the VDR and the RXR that define interactions with the nuclear import and export machinery and showed that the effect of mutations impairing VDR import is compensated by increased expression of RXR. Our second objective was to elucidate mechanisms that regulate the subcellular targeting and metabolism of the hormone, calcitriol. First, we studied subcellular targeting of calcitriol with microscopy. These experiments relied on using our biologically active fluorescing derivatives of calcitriol together with green fluorescent protein chimeras of heat shock proteins (hsp), which were generated by our collaborators Dr. Adams and colleagues at Cedar-Sinai Medical Center. We found that overexpression of hsp70 family of proteins increased cellular uptake of calcitriol. Moreover, mutational analysis demonstrated that members of the hsp70 family of proteins play important roles in calcitriol endocytosis, targeting to the endoplasmic reticulum and the mitochondria. In another study, we explored the roles of histamine in the regulation of mitochondrial calcitriol synthesis and catabolism using a histamine deficiency knockout mouse model. This histamine deficient mice was generated by targeted deletion of the gene encoding the histamine synthesizing enzyme, histidine decarboxylase (HDC-/-). HDC-/- mice displayed undetectable tissue histamine levels, impaired gastric acid secretion, impaired passive cutaneous anaphylaxis, and decreased mast cell degranulation. Detailed characterization of the bone phenotype revealed that HDC-/- mice have higher bone mineral density, increased cortical bone thickness and mineralization, higher rate of bone formation and a marked decrease of osteoclast number when compared to wild-type (WT) mice. After ovariectomy, bone loss was significantly reduced in HDC-/- mice. Using this mouse model, we demonstrated for the fist time the role of histamine in the regulation of serum calcitriol concentrations. Histamine deficiency increased serum calcitriol levels by increasing renal expression of the calcitriol synthesizing enzyme, 25-hydroxyvitamin D3-1a-hydroxylase and reduced the expression of the calcitriol catabolizing enzyme, 25-hydroxyvitamin D3-24-hydroxylase. As a result of elevated serum calcitriol concentrations, serum parathyroid hormone levels were suppressed, serum calcium and phosphorus concentrations increased. This excess calcitriol increased the serum levels of alkaline phosphatase, a marker of osteoblastic activity. In the same time, osteoclasts were unresponsive to the effects of this excess calcitriol, leading to a net gain in bone mass in HDC-/- mice. Our findings suggested that conditions leading to increased serum and tissue histamine levels could negatively regulate vitamin D and calcitriol homeostasis. Our ongoing studies explore the roles of Klotho protein in the regulation of calcitriol targeting and metabolism. It is well established that selective resistance to the anticancer effects of calcitriol can develop. Our third objective was to understand the mechanism of growth regulation by calcitriol and reveal the mechanisms responsible for hormone responsiveness. Our studies in rat osteosarcoma cells demonstrated that accelerated degradation of RXR and the expression of a dominant negative truncated RXR could cause resistance to the growth inhibitory effects of calcitriol. We then synthesized and purified calcitriol antagonists (BCA11 and BCA21), and found that similarly to VDR agonists they inhibited proliferation of cancer cell lines and the growth of human breast cancer xenografts in nude mice (US provisional patent No. 60/300,909; filed June 22, 2001; NIH reference No. E-213/01/0). Using GST pull-down experiments and microscopy, we found that both VDR agonists and antagonists induce the release of the corepressor NCoR from VDR, and this leads to the export of factors from the nucleus to the mitochondria, where they cause apoptosis. The presence or absence of these factors defines, at least in part, the sensitivity of cells to the growth inhibitory effects of vitamin D derivatives. In conclusion, we made major progress in all three areas of our research. Our findings lead to a better understanding of diseases associated with insufficient or excess calcitriol actions.